The present application claims priority to Japanese Application No. P11-196745 filed Jul. 9, 1999; which application is incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
The present invention relates to a magneto-optical recording medium for an optical system having a wavelength/numerical aperture (NA) of lens of not more than about 1000 nm and a method of producing the same, more particularly a magneto-optical recording medium suppressed in temperature rise and improved in the heat characteristics without reducing the optical characteristics and a method of producing the same.
2. Description of the Related Art
An ordinary magneto-optical recording medium, for example, a magneto-optical disk is structured by, as shown in a cross-sectional view of FIG. 1, for example a light transmitting substrate 1 on which are successively stacked a light transmitting first dielectric layer 2 comprised of SiN of a thickness of 80 nm to 100 nm, a magneto-optical recording layer 3 comprised of for example TbFeCo, a light transmitting second dielectric layer 4 comprised of for example SiN of a thickness of 20 nm to 40 nm, and a metal reflective layer 5 comprised of Al of a thickness of 50 nm to 60 nm. In the magneto-optical recording medium of the structure shown in FIG. 1, the light L strikes from the substrate 1 side and the light L is mainly reflected at the interface between the second dielectric layer 4 and the metal reflective layer 5. A reflected light of the light L is detected by a detector and converted into an electric signal.
The magneto-optical recording layer 3 is made light transmitting by setting the thickness to not more than 30 nm, for example, 15 nm to 25 nm. Since an enhancement effect of the Kerr effect can be obtained by using multiple interference between the magneto-optical recording layer 3 and the metal reflective layer 5, the magneto-optical recording layer 3 has the advantage that the reflectance R can be lowered with the optical performance index, that is, Rxc2x7xcex8K (R is the reflectance, xcex8K is the Kerr rotation angle) kept constant. Also, if the combination of the magneto-optical recording layer 3 and the metal reflective layer 5 is suitably selected, it is possible to prevent a temperature rise in the magneto-optical recording layer 3 and design for the optimal heat characteristics.
Below, the optical aspects and heat aspects of the above conventional magneto-optical recording medium will be explained in detail.
First, the optical aspects of the above magneto-optical recording medium will be explained. If the optical performance index is fixed when changing the film configuration, the amplitude of the signal and disk noise are kept at a constant level and the S/N does not change. In a magneto-optical disk, shot noise is predominant in a high frequency region. Also, since the shot noise level is proportional to the square root of a light quantity detected on a detector detecting the signal, the lower the reflectance, the lower the shot noise level. Therefore, the lower the reflectance can be made in the design within the range which the heat characteristics and the servo characteristics can allow, the more the SIN, especially the high-frequency characteristics, are improved.
A decrease of reflectance can be realized by coating a SiN dielectric layer and a thick magneto-optical recording layer on a substrate. However, in this case, even if the thickness of the SiN dielectric layer is changed, the reflectance will not become lower than 20%. To lower the reflectance further, a configuration making the thickness of the magneto-optical recording layer not more than 30 nm and arranging a reflective layer via the dielectric layer is effective. Particularly, when the thickness of the magneto-optical recording layer is made not more than 20 nm, the transmittance of light increases. Therefore, it becomes possible to change the reflectance in a wide range by using multiple interference between the magneto-optical recording layer and the reflective layer. As the reflective layer, a metal of a high reflectance mainly comprised of Al is usually used.
Next, the heat aspects of the above magneto-optical recording medium will be explained. From the heat aspects, the heat characteristics of the magneto-optical recording layer are controlled by suitably selecting the thicknesses of the metal reflective layer and the second dielectric layer.
However, according to the conventional magneto-optical recording medium having the above film configuration, it is difficult to sufficiently control the heat characteristics.
Along with the increasing higher density of magneto-optical recording media in these years, the spot size of light has been made smaller in diameter. Therefore, when raising the intensity of the reproducing light and repeatedly reproducing information, the recording marks are sometimes lost and information recorded on the medium erased. Especially when using a light source of a short wavelength or an optical system having an NA over 0.7, the photo-absorption concentrates at the surface of the magneto-optical recording layer. Thus the temperature of the magneto-optical recording medium including the magneto-optical recording layer rises before the heat reaches the metal reflective layer and disperses.
Generally, in a metal film, when reducing its thickness, the heat conductivity decreases due to the decrease in the mean free path of electrons. It is known that the heat conductivity drastically decreases when the thickness is made about 30 nm or less. In a magneto-optical recording layer comprised of TbFeCo or another metal layer as well, when the thickness of the magneto-optical recording layer is made 30 nm or less as described above in order to maintain the optical characteristics, the rise of temperature due to decrease in the heat conductivity becomes a problem.
As described above, in a magneto-optical recording medium, there is a problem that it is difficult to achieve both of maintenance of the optical performance index and prevention of temperature rise.
Further, when using a light source of a short wavelength of not more than 490 nm and/or an optical system having an NA over 0.7, the rise of temperature in the magneto-optical recording layer becomes remarkable. Therefore, when the second dielectric layer is formed thinly at a thickness of for example about 10 nm, the dispersion of heat from the magneto-optical recording layer becomes insufficient. Then, the problem rises in that the heat control of the magneto-optical recording layer becomes impossible.
The present invention was made in consideration of the above problem. Therefore, the present invention has as its object to provide a magneto-optical recording medium maintained in the optical performance index and improved in the heat characteristics and a method of producing the same.
To achieve the above object, the magneto-optical recording medium of the present invention is a magneto-optical recording medium for recording and reproduction of information by focusing of light of a microspot diameter, characterized by comprising a first light transmitting dielectric layer to a first surface of which the light strikes; a magneto-optical recording layer formed on a second surface opposed to the first surface of the first dielectric layer; a light transmitting metal layer formed on the magneto-optical recording layer to a predetermined thickness required for dispersion of heat from the magneto-optical recording layer and having a larger heat conductivity than the magneto-optical recording layer; a second light transmitting dielectric layer formed on the light transmitting metal layer; and a metal reflective layer formed on the second dielectric layer and reflecting light.
The magneto-optical recording medium of the present invention preferably is characterized in that the light of a microspot diameter is light from an optical system of which a ratio of (wavelength)/(numerical aperture (NA) of lens) is not more than about 1000 nm. Also, the magneto-optical recording medium of the present invention preferably is characterized in that the predetermined thickness of the light transmitting metal layer is not less than about 10 nm.
The magneto-optical recording medium of the present invention preferably is characterized in that the upper limit of the thickness of the light transmitting metal layer is set within a range where at least part of the light striking the light transmitting metal layer from the first surface side passes through the light transmitting metal layer to strike the second dielectric layer.
The magneto-optical recording medium of the present invention further preferably is characterized in that the upper limit of the thickness of the light transmitting metal layer is about 60 nm.
Also, the magneto-optical recording medium of the present invention preferably is characterized in that the light transmitting metal layer is comprised of a metal having a large reflectance with respect to light. The magneto-optical recording medium of the present invention further preferably is characterized in that the light transmitting metal layer contains Al. Alternatively, the magneto-optical recording medium of the present invention further preferably is characterized in that the light transmitting metal layer contains Ag.
The magneto-optical recording medium of the present invention preferably is characterized by further comprising a substrate on the first surface of the first dielectric layer. Alternatively, the magneto-optical recording medium of the present invention preferably is characterized by further comprising a substrate at the surface of the metal reflective layer at the side opposite to the side which the light strikes.
Due to this, it becomes possible to promote effective dispersion of heat from the magneto-optical recording layer and it is possible to improve the heat characteristics of a mageto-optical recording medium. According to the magneto-optical recording medium of the present invention, a light transmitting metal layer having a thickness of 10 nm or more is formed between the magneto-optical recording layer and the metal reflective layer next to the magneto-optical recording layer. Then, the magneto-optical recording layer and the light transmitting metal layer share conductive electrons in charge of conduction of heat and the substantive heat conductivity of the magneto-optical recording layer increases. Therefore, the local temperature rise of the magneto-optical recording layer due to focusing of light is suppressed.
Generally, when making a metal film thinner, due to the decrease of the mean free path of electrons, the heat conductivity decreases. However, according to the magneto-optical recording medium of the present invention, the thickness of the light transmitting metal layer can be made greater within the range where there is light transmissiveness. Therefore, it is possible to raise further the heat conductivity of the magneto-optical recording layer by thickening the light transmitting metal layer. Also, this effect becomes more effective the higher the heat conductivity of the light transmitting metal layer.
Also, according to the magneto-optical recording medium of the present invention, by using multiple interference between the light transmitting magneto-optical recording layer and the metal reflective layer, the optical performance index of Rxc2x7xcex8K (R is the reflectance, xcex8K is the Kerr rotation angle) can be kept constant. Therefore, when forming the light transmitting metal layer having the above thickness, the optical characteristics of the magneto-optical recording layer are not impaired.
To achieve the above object, the method of producing a magneto-optical recording medium of the present invention is a method of producing a magneto-optical recording medium for recording and reproduction of information by focusing of light of a short wavelength or high NA, said method comprising the steps of forming a light transmitting first dielectric layer on the surface of the light transmitting substrate at the side opposite to the side which the light strikes; forming a magneto-optical recording layer on the first dielectric layer; forming on the mangneto-optical recording layer a light transmitting metal layer having a predetermined thickness required for dispersion of heat from the magneto-optical recording layer and a larger heat conductivity than the magneto-optical recording layer; forming a light transmitting second dielectric layer on the light transmitting metal layer; and forming a metal reflective layer reflecting light on the second dielectric layer.
The method of producing a magneto-optical recording medium of the present invention preferably is characterized in that the steps of forming the first dielectric layer, the magneto-optical recording layer, the light transmitting metal layer, the second dielectric layer, and the metal reflective layer comprise forming layers by sputtering.
The method of producing a magneto-optical recording medium of the present invention further preferably is characterized in that each of the sputtering steps is performed in the same apparatus.
The method of producing a magneto-optical recording medium of the present invention preferably is characterized by further comprising the step of flattening a surface of the light transmitting metal layer after the step of forming the light transmitting metal layer. The method of producing a magneto-optical recording medium of the present invention further preferably is characterized in that the step of flattening the surface of the light transmitting metal layer comprises the step of reverse sputtering. The method of producing a magneto-optical recording medium of the present invention preferably is characterized in that the steps of forming the first dielectric layer, the magneto-optical recording layer, the light transmitting metal layer, the second dielectric layer, and the metal reflective layer comprise forming layers by sputtering, and the reverse sputtering is performed in the same apparatus as the sputtering steps.
According to the method of producing a magneto-optical recording medium of the present invention, since it is possible to continuously form the first dielectric layer, the magneto-optical recording layer, the light transmitting metal layer, the second dielectric layer, and the metal reflective layer on the substrate, only the step of forming the light transmitting metal layer has to be added to the conventional production steps of a magneto-optical recording medium. That is, the production steps are not complicated compared with the conventional method of production and no new facilities etc. are required.
Further, when flattening the surface of the light transmitting metal layer by reverse sputtering in the method of producing a magneto-optical recording medium of the present invention, it is possible to improve the surface condition of the light transmitting metal layer with a simple step and decrease noise due to roughness of the surface.
Also, to achieve the above object, the method of producing a magneto-optical recording medium of the present invention is a method of producing a magneto-optical recording medium for recording and reproduction of information by focusing of light of a short wavelength or high NA, comprising the steps of forming a metal reflective layer reflecting the light on a surface of the substrate at the side which the light is focused on; forming a light transmitting second dielectric layer on the metal reflective layer; forming on the second dielectric layer a light transmitting metal layer having a predetermined thickness required for dispersion of heat from a magneto-optical recording layer and a larger heat conductivity than the magneto-optical recording layer; forming the mageto-optical recording layer on the light transmitting metal layer; and forming a light transmitting first dielectric layer on the magneto-optical recording layer.
The method of producing a magneto-optical recording medium of the present invention preferably is characterized in that the steps of forming the metal reflective layer, the second dielectric layer, the light transmitting metal layer, the magneto-optical recording layer, and the first dielectric layer comprise forming layers by sputtering. The method of producing a magneto-optical recording medium of the present invention further preferably is characterized in that each of the sputtering steps is performed in the same apparatus.
The method of producing a magneto-optical recording medium of the present invention preferably is characterized by comprising the step of flattening a surface of the light transmitting metal layer after the step of forming the light transmitting metal layer. The method of producing a magneto-optical recording medium of the present invention preferably is characterized in that the step of flattening the surface of the light transmitting metal layer comprises the step of reverse sputtering. The method of producing a magneto-optical recording medium of the present invention preferably is characterized in that the steps of forming the first dielectric layer, the magneto-optical recording layer, the light transmitting metal layer, the second dielectric layer, and the metal reflective layer comprise forming layers by sputtering, and the reverse sputtering is performed in the same apparatus as the sputtering steps.
According to the method of producing a magneto-optical recording medium of the present invention, since it is possible to continuously form the metal reflective layer, the second dielectric layer, the light transmitting metal layer, the magneto-optical recording layer, and the first dielectric layer on the substrate, only the step of forming the light transmitting metal layer has to be added to the conventional production steps of a magneto-optical recording medium. That is, the production steps are not complicated compared with the conventional method of production and no new facilities etc. are required.
Further, when flattening the surface of the light transmitting metal layer by reverse sputtering in the method of producing a magneto-optical recording medium of the present invention, it is possible to improve the surface condition of the light transmitting metal layer with a simple step and decrease noise due to roughness of the surface.